This invention pertains in general to image transfer technology. More particularly, this invention relates to pulse width modulation of multiple lasers within an image forming device such as a laser printer.
Pursuant to laser printer technology, a latent image is created on a surface of an insulating, photo-conducting material by selectively exposing areas of the surface to light. For the case of a laser printer, the surface is in the form of a rotating drum. A difference in electrostatic charge density is created between areas on the surface depending on the degree to which such areas are exposed to light. A visible image is then developed on the drum using one or more types of electrostatic toner. For the case of black and white printing, a single, black toner is used. For the case of color printing, multiple different color toners are used. Each toner is selectively attracted onto the photoconductive surface of the drum either exposed or unexposed to light, depending on the relative electrostatic charges on the photoconductive surface, characteristics of the development toner, and the type of toner used. Depending on the particular implementation, the photoconductive surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
The developed image is then transferred from the drum surface onto a sheet of paper. More particularly, a transfer roller is imparted with an electrostatic charge that is opposite to that of the toner. The transfer roller is rotated in proximity with the photoconductive surface of the drum. The transfer roller pulls the toner from the photoconductive surface, transferring the toner onto a charged sheet of paper. The transferred toner maintains the pattern of the image that was developed on the photoconductive surface.
In operation, a laser printer scans a laser beam horizontally across the photosensitive, electrically charged drum. By modulating the laser beam, resulting variations in charge will impart proportionate amounts of toner being deposited onto a sheet of paper.
More particularly, laser printers print pages of information onto individual sheets of paper by applying a particular toner, such as black toner for the case of gray scale printing, to selected small regions of fixed size referred to as pixels. By placing toner in only a portion of a pixel region, it is possible to create the effect of shades of gray. One technique for placing toner in only a portion of a pixel region uses a pulse width modulation (PWM) technique.
Complicating matters, new laser printers, including color laser printers, have attempted to incorporate more than one laser beam to scan the photosensitive drum that collects the toner in specific locations. Such multiple laser beams enable faster through-put while maintaining or increasing resolution.
However, a problem occurs if the lasers have different optical paths to the photosensitive drum because the pixel width becomes a function of the optical path. As a result, it becomes extremely difficult to ensure that both, or all, lasers will generate pixels of the same size and line alignment on the drum.
Laser printers are distinguished from other types of printers by their ability to place precise amounts of toner into very small regions of a page at relatively high speed. As a result, laser printers generate image quality that is far greater than most other types of printers. However, laser printers operate by scanning a photoconductive drum upon which a rendered image is held. This results in an intrinsic quantization of the image in the vertical direction of the page. Additionally, there exist limitations in circuitry that is used to modulate the horizontal scanning. These limitations result in quantization of the image so that a single cell, or pixel, is effectively formed. If pixels are made small enough, the quantization effects can be made imperceptibly small to the human eye. However, there are practical limits. First, the vertical quantization is limited by the scan rate and the speed with which the photoconductive drum is rotated. Secondly, horizontal quantization is limited by the ability to transfer data in serial form to the scanning laser. The horizontal quantization limits the number of transitions that modulate the scanning laser, thereby limiting the density of horizontal dots that are placed onto a printed page.
In an effort to increase the resolution capability of laser printers, various techniques have been used to increase the number of horizontal dot components of a laser. Irrespective of the technique used to generate horizontal dot components, the laser needs to be phase locked to a single signal edge referred to as a beam detect. The beam detect provides a reference signal that indicates when the scanning laser has begun to sweep across the photosensitive drum.
In operation, a pixel clock is phase locked to the beam detect signal. One technique uses a clock generator as described in U.S. Pat. Nos. 5,438,353 and 5,760,816 listing Applicant as the inventor, and describing a circuit providing one such clock generator. Such U.S. Pat. Nos. 5,438,353 and 5,760,816 are herein incorporated by reference.
More particularly, the error in dot placement using a clock generator described above on pixel boundaries can be substantially improved by using a phase adjusted pulse width modulator. See U.S. patent application Attorney Docket No. 10990775-1, entitled xe2x80x9cPhase Adjusted Pulse Width Modulatorxe2x80x9d, naming the inventor as Robert D. Morrison, which application describes one technique using phase locked pulse width modulation (PWM). Such U.S. Pat. Application Attorney Docket No. 10990775-1, is herein incorporated by reference. A standard phase locked loop cannot be used in this environment because it is not possible to generate an error signal that would correct the pixel dot clock. More particularly, there is only one beam detect edge that the pixel dot clock could lock phase to, and that phase is required to be rigidly maintained for the length of a scanned line.
Several recent attempts have been made to provide a laser printer that uses two laser diodes in order to reduce the mechanical scanning rotation rate. However, the above requirements become very difficult to meet when multiple lasers are used to scan a photosensitive drum. Currently, laser printers are pushing the limit of practical rotation rates for a scanner. Thus, a laser printer is able to run faster by slowing down the scanner, and scanning a pair of beams in parallel. Since the lasers are currently very close to each other such that the lasers tend to follow the same optical scan path, the same video frequency can be used for both laser modulation signals in order to generate pixels. However, the implementation of multiple lasers, particularly for color printing, necessitates the use of different laser optical scan paths that sweep different drum locations.
More particularly, when rendering solutions for color printer technology it is desirable to provide laser optical scan paths that sweep across significantly different drum locations to allow different colors to be deposited. If the lasers are positioned right next to each other, there would be no way to determine which scan would attract which color toner. In such situation, the optical path is significantly different, and attempts to ensure equal optical paths tend to be completely impractical or unrealizable. As a result, video frequency and pixel edge placement for each laser becomes difficult, if not impossible using presently available techniques.
Additionally, for single-color or gray-scale printing where multiple lasers have different optical paths, similar problems result and prior art solutions are inadequate or unacceptable.
Therefore, there exists a need for improved techniques for implementing multiple lasers on image forming devices.
A multiple laser system and method uses multiple optical laser paths and processing circuitry, such as multiple clock independent pulse width modulators, to generate a video signal for each laser in a manner that accommodates different video frequencies for each optical path. As a result, it is ensured that each optical scan path for each laser on a photosensitive drum will produce pixels of substantially identical width and location by ensuring that the laser modulation occurs with an appropriate frequency for each specific laser path.
According to one aspect, a system is provided for rendering an output image on an image transfer surface using multiple optical scan paths in an image rendering device. The system includes a plurality of lasers, a plurality of scan time detectors, and processing circuitry. Each of the lasers is configured to generate an optical scan path on the surface. Each scan time detector is associated with a corresponding laser and is operative to measure scan time of the laser along the respective optical scan path. The processing circuitry communicates with each scan time detector and is operative to generate a video signal having a video frequency associated with the scan time detected for the respective laser.
According to another aspect, an image forming device is provided for rendering an output image on a surface of a photoconductive drum using multiple optical laser paths. The image forming device includes a laser print engine having an image forming surface provided on a photoconductive drum. The laser print engine further includes a pair of lasers, a pair of scan time detectors, and a pair of pulse width modulators. Each laser is configured to generate a unique optical scan path on the drum surface. Each scan time detector is associated with a corresponding laser and is operative to measure the time it takes the laser to sweep across the associated optical scan path. Each pulse width modulator communicates with each laser and is operative to generate a video signal having a video frequency associated with the scan time detected for the respective laser.
According to yet another aspect, a method is provided for rendering an image onto an image transfer surface using multiple lasers. The method includes: providing a plurality of lasers each configured to generate an optical laser scan path onto an image transfer surface; detecting the time each laser takes to sweep the respective scan path; determining a unique video frequency for each scan path based on the respective sweep time; and driving each laser with the unique video frequency such that pixels will be produced by the plurality of lasers having substantially identical width and location.